1. Field of the Invention
The present invention relates to a multilayer thin film pattern and a method of manufacturing a display device.
2. Description of Related Art
For liquid crystal displays, there are for example a transmissive type, a reflective type and a transflective type liquid crystal display (for example see Japanese Unexamined Patent Application Publication No. 11-101992). A transmissive liquid crystal display shows an image by a backlight disposed at its back. A reflective liquid crystal display shows an image by its surrounding light reflected on a reflector surface which is disposed to a substrate. Therefore, the reflective liquid crystal display includes a pixel electrode for reflecting light, that is, a reflective pixel electrode. A transflective liquid crystal display transmits a part of light and reflects a part of light. In the transflective liquid crystal display, a TFT array substrate has both a transmissive pixel electrode (transmissive electrode) and a reflective pixel electrode.
In the transmissive liquid crystal display, it is usually required that both a pixel electrode over a TFT array substrate and an opposing electrode over a color filter substrate are transparent, and a transparent conductive film, such as ITO, is used for both of them as an electrode material. Accordingly, at the time of the alternating current drive of a liquid crystal, the abovementioned pixel electrode and the opposing electrode can apply positive and negative voltages to the liquid crystal under almost the same conditions. On the other hand, in the transflective liquid crystal display, a metal film such as Al is used as a reflective pixel electrode (reflective electrode). Therefore, display flicker and liquid crystal image sticking are generated according to the driving condition due to a work function difference with the transparent conductive film which is the opposing electrode.
As a countermeasure for such flicker and image sticking, a technique for forming a transparent conductive film made of the same material as an opposing electrode over a metal of a reflective electrode is disclosed in Japanese Unexamined Patent Application Publication Nos. 2003-255378 and 2005-275323. In Japanese Unexamined Patent Application Publication No. 2005-275323, the metal film of the reflective electrode and the transparent conductive film over the reflective electrode are collectively wet etched using the same etchant and the same mask pattern. This enables to form the reflective electrode and the transparent conductive film which has the same pattern shape as the reflective electrode.
Moreover, in the transflective liquid crystal display, both processes of forming the transmissive electrode and forming the reflective electrode are included. Therefore, in the transflective liquid crystal display, there are more photolithography processes required compared with the transmissive liquid crystal display and the reflective liquid crystal display. In order to reduce the number of photolithography processes, a technique of devising the photolithography is disclosed in Japanese Unexamined Patent Application Publication No. 2005-215277. In Japanese Unexamined Patent Application Publication No. 2005-215277, using a gray tone or a halftone exposure technique for the photolithography, the film thickness of the resist (photosensitive resin) pattern is changed for each part. By the resist pattern with different film thicknesses, the transmissive electrode and the reflective electrode are processed in one photolithography process.
However, when the metal film and the transparent conductive film formed thereover are collectively wet etched by the technique disclosed in Japanese Unexamined Patent Application No. 2005-275323, the transparent conductive film may hang over the metal film edge face and remain as a protruding shape (an overhang). FIGS. 8A to 8D are cross-sectional diagrams schematically showing a manufacturing process of a TFT array substrate in a transflective liquid crystal display according to Japanese Unexamined Patent Application Publication No. 2005-275323. In FIG. 8A, a first transparent conductive film 2 is formed and patterned over an interlayer insulating film 1 which is formed above a TFT (not shown), a scanning signal line (not shown) and a display signal line (not shown). Next, in FIG. 8B, a metal film 3 as a reflective electrode is formed to cover the first transparent conductive film 2. Then, a second transparent conductive film 4 is formed to prevent flicker and image sticking. Subsequently, a resist pattern 5 of desired shape is formed over the second transparent conductive film 4. The metal film 3 and the second transparent conductive film 4 are collectively wet etched with the resist pattern 5 being formed thereover, as shown in FIG. 8C. After that, the resist pattern 5 is removed and it becomes the configuration as shown in FIG. 8D.
When a multilayer thin film pattern is formed by the above method, the second transparent conductive film 4 which is the upper layer is formed to be protruding shape projected from the pattern end of the lower layer metal film 3. Especially when using isotropic etching such as wet etching, the protruding shape of the second transparent conductive film 4 is likely to appear. FIG. 9 is a partial enlarged diagram showing the state during the collective wet etching process of the second transparent conductive film 4 and the metal film 3. In isotropic etching, etching proceeds to the vertical and the horizontal directions at the same time. That is, while the etching of the second transparent conductive film 4 and the metal film 3 proceeds in the direction of film thickness, the etching also proceeds in the direction perpendicular to film thickness (side etch). By the side etch, as shown in FIG. 9, a hollow 6 is formed between an etching surface 7 of the metal film 3 and the resist pattern 5 surface during the etching. Since the etching proceeds also from this hollow 6, the protrusion is likely to be generated. Furthermore, generally the etching rate of the metal film 3 is faster than that of the second transparent conductive film 4, thus the metal film 3 is more likely to be side etched than the second transparent conductive film 4. Consequently, the second transparent conductive film 4 projects to form the protruding shape.
Thus, if the protrusion is formed in the edge face of the multilayer thin film, during the process of rubbing substrates such as a rubbing process in the next panel manufacturing process, the protruding portion comes off to be a cause of generating a contamination particle. Then, the piece of the separated film could lead to a short-out between adjacent pixels or between a pixel electrode and an opposing electrode, thereby causing a display failure. Moreover, when the edge face of the multilayer film pattern is covered with a covering film, continuity of the covering film is broken at the protruding portion. If the covering film is a protective insulating film, an insulation failure occurs in the portion of the discontinuity. If the covering film is a conductive film, an electric connection failure occurs in the portion of the discontinuity. As the definition of display devices advances higher in the future, pixel size and space between adjacent pixels will become smaller. Thus the process to more effectively prevent from generating the projection is required.
Moreover, in Japanese Unexamined Patent Application Publication No. 2005-215277, the resist pattern with different film thicknesses is used to form the transparent electrode and the reflective electrode. The thinner part of the resist pattern with different film thicknesses is removed by ashing. Oxygen plasma treatment is usually used for this ashing. However, according to the method of Japanese Unexamined Patent Application Publication No. 2005-215277, the ashing is performed while the transparent conductive film is exposed to the surface, and thus abnormal electrical discharge may arise. By abnormal electrical discharge, not only the transparent conductive film but the interlayer insulating film provided thereunder is also damaged. Furthermore, it may cause a failure such as a disconnection of the line provided in the lower layer.
The present invention is made in order to solve the above problems and an object of this invention is to provide a manufacturing method of a multilayer thin film pattern and a display device that can easily obtain a pattern with a desired shape.